Generally, in a bulldozer, an engine drives a hydraulic pump and the bulldozer is operated to perform work by using the hydraulic power, while at the same time, a sprocket is driven via a torque converter, a transmission, and a final speed reducer to travel the bulldozer.
FIG. 4 is a hydraulic control circuit diagram for operating a blade of a conventional bulldozer; only a lift circuit for a blade 66 is shown, other control circuits being omitted.
A delivery circuit of a fixed-delivery hydraulic pump 60, driven by an engine 1, is connected to lift cylinders 63, 63 via a lift operating valve 61, which is equipped with an operating lever 62 for moving the blade 66 upwardly or downwardly. Reference numeral 64 denotes an oil tank.
The lift operating valve 61 shown in FIG. 4 is set to position (B) while not in operation and the blade 66 is in a stationary state.
When the lift operating valve 61 is set to position (A) by operating the operating lever 62, the respective lift cylinders 63, 63 are contracted, causing the blade 66 to move upwardly.
When the lift operating valve 61 is set to position (C), the respective lift cylinders 63, 63 are extended to push the blade 66 downwardly.
To perform excavation or earth carrying work, the blade 66 is pushed downwardly to excavate earth surface, while moving the blade upwardly and downwardly as the bulldozer is advanced; when the blade 66 is filled with earth, the blade 66 is placed in the stationary state, and the bulldozer is advanced to carry the earth.
The operating horsepower for the blade 66 is large; it reaches 40% of the engine horsepower when the hydraulic circuit of the working machine is in a relief state. The then actual horsepower supplied to the sprocket is approximately 30%.
Hence, as the load of the blade 66 increases, the horsepower supplied to the sprocket decreases, resulting in a drop in the vehicle speed. The operator senses the drop in the engine revolution or the drop in the vehicle speed, and lifts the blade 66 to reduce the load applied to the blade 66, thereby recovering the vehicle speed.
To solve the problem, the fixed-delivery hydraulic pump 60 shown in FIG. 4 has been replaced by a variable-delivery hydraulic pump 65 as shown in FIG. 5. Specifically, the load pressure at the respective lift cylinders 63, 63 of the blade 66 is detected and the delivery flow rate of the variable-delivery hydraulic pump 65 is controlled according to the detected load pressure so as to prevent wasteful oil from being discharged.
According to the configuration shown in FIG. 4, however, the full quantity of the fixed-delivery hydraulic pump 60 is always discharged while the blade 66 is being operated, consuming the horsepower. For instance, when shifting from a push-down excavation mode on a downhill to a push-up earth carrying mode on an uphill, if the lifting operation is implemented with the blade 66 filled with earth, then the operating horsepower of the blade 66 increases, causing a decrease in the engine output distributed to the traveling system. As a result, the vehicle speed drops and the speed of the engine 1 drops.
Further, if the blade 66 is excessively filled with earth and sand, then the apparent weight of the vehicle increases, causing the torque converter to be stalled. This results in an insufficient tractive output and the vehicle stops advancing.
On the other hand, according to the configuration shown in FIG. 5, the delivery flow rate of the variable-delivery hydraulic pump 65 is controlled to prevent wasteful oil from being discharged; however, consideration has not been given to increasing the engine output distributed to the traveling system so as to prevent the torque converter from being stalled.